Clinker bricks are partially vitrified bricks used in the construction of buildings. Clinkers are burnt under temperatures so high that the pores of the fuel property are closed by the beginning sinter process and are often mis-shapen. Thus they are considerably denser and therefore heavier and more irregular than regular bricks. Clinkers hardly take up water and are very durable, but have higher thermal conductivity than the more porous conventional red bricks, thus lending less insulation to climate-controlled structures. The name comes from the sound they make when banged together.